Maxim L. Khodachenko

Determining the mass loss limit for close-in exoplanets: what can we learn from transit observations?

Aims. We study the possible atmospheric mass loss from 57 known transiting exoplanets around F, G, K, and M-type stars over evolutionary timescales. For stellar wind induced mass loss studies, we estimate the position of the pressure balance boundary between Coronal Mass Ejection (CME) and stellar wind ram pressures and the planetary ionosphere pressure for non- or weakly magnetized gas giants at close orbits. Methods. The thermal mass loss of atomic hydrogen is calculated by a mass loss equation where we consider a realistic heating efficiency, a radius-scaling law and a mass loss enhancement factor due to stellar tidal forces. The model takes into account the temporal evolution of the stellar EUV flux by applying power laws for F, G, K, and M-type stars. The planetary ionopause obstacle, which is an important factor for ion pick-up escape from non- or weakly magnetized gas giants is estimated by applying empirical power-laws. Results. By assuming a realistic heating efficiency of about 10–25% we found that WASP-12b may have lost about 6–12% of its mass during its lifetime. A few transiting low density gas giants at similar orbital location, like WASP-13b, WASP-15b, CoRoT-1b or CoRoT-5b may have lost up to 1–4% of their initial mass. All other transiting exoplanets in our sample experience negligible thermal loss (≤1%) during their lifetime. We found that the ionospheric pressure can balance the impinging dense stellar wind and average CME plasma flows at distances which are above the visual radius of “Hot Jupiters”, resulting in mass losses <2% over evolutionary timescales. The ram pressure of fast CMEs cannot be balanced by the ionospheric plasma pressure for orbital distances between 0.02–0.1 AU. Therefore, collisions of fast CMEs with hot gas giants should result in large atmospheric losses which may influence the mass evolution of gas giants with masses <MJup. Depending on the stellar luminosity spectral type, planetary density, heating efficiency, orbital distance, and the related Roche lobe effect, we expect that at distances between 0.015–0.02 AU, Jupiter-class and sub-Jupiter-class exoplanets can lose several percent of their initial mass. At orbital distances ≤0.015 AU, low density hot gas giants in orbits around solar type stars may even evaporate down to their coresize, while low density Neptune-class objects can lose their hydrogen envelopes at orbital distances ≤0.02 AU.

Observation of Kink Instability During Small B5.0 Solar Flare on 2007 June 4

Using multi-wavelength observations of SOHO/MDI, SOT-Hinode/blue-continuum (4504 ?), G band (4305 ?), Ca II H (3968 ?), and TRACE 171 ?, we present the observational signature of a highly twisted magnetic loop in AR?10960 during the period 04:43?UT-04:52?UT on 2007 June 4. SOT-Hinode/blue-continuum (4504 ?) observations show that penumbral filaments of positive polarity sunspot have counterclockwise twist, which may be caused by the clockwise rotation of the spot umbrae. The coronal loop, whose one footpoint is anchored in this sunspot, shows strong right-handed twist in chromospheric SOT-Hinode/Ca II H (3968 ?) and coronal TRACE 171 ? images. The length and the radius of the loop are L ~ 80?Mm and a ~ 4.0?Mm, respectively. The distance between neighboring turns of magnetic field lines (i.e., pitch) is estimated as 10?Mm. The total twist angle, ? ~ 12? (estimated for the homogeneous distribution of the twist along the loop), is much larger than the Kruskal-Shafranov instability criterion. We detected clear double structure of the loop top during 04:47?UT-04:51?UT on TRACE 171 ? images, which is consistent with simulated kink instability in curved coronal loops. We suggest that the kink instability of this twisted magnetic loop triggered a B5.0 class solar flare, which occurred between 04:40?UT and 04:51?UT in this active region.

Collisional and viscous damping of MHD waves in partially ionized plasmas of the solar atmosphere

Magnetohydrodynamic (MHD) waves are widely considered as a possible source of heating for various parts of the outer solar atmosphere. Among the main energy dissipation mechanisms which convert the energy of damped MHD waves into thermal energy are collisional dissipation (resistivity) and viscosity. The presence of neutral atoms in the partially ionized plasmas of the solar photosphere, chromosphere and prominences enhances the efficiency of both these energy dissipation mechanisms. A comparative study of the efficiency of MHD wave damping in solar plasmas due to collisional and viscous energy dissipation mechanisms is presented here. The damping rates are taken from Braginskii 1965 and applied to the VAL C model of the quiet Sun (Vernazza et al. 1981). These estimations show which of the mechanisms are dominant in which regions. In general the correct description of MHD wave damping requires the consideration of all energy dissipation mechanisms via the inclusion of the appropriate terms in the generalized Ohms law, the momentum, energy and induction equations. Specific forms of the generalized Ohms Law and induction equation are presented that are suitable for regions of the solar atmosphere which are partially ionised.

Probing the blow-off criteria of hydrogen-rich ‘super-Earths’

The discovery of transiting “super-Earths” with inflated radii and known masses such as Kepler-11b-f, GJ 1214b and 55 Cnc e, indicates that these exoplanets did not lose their nebula-captured hydrogen-rich, degassed or impact-delivered protoatmospheres by atmospheric escape processes. Because hydrodynamic blow-off of atmospheric hydrogen atoms is the most efficient atmospheric escape process we apply a time-dependent numerical algorithm which is able to solve the system of 1-D fluid equations for mass, momentum, and energy conservation to investigate the criteria under which “super-Earths” with hydrogen-dominated upper atmospheres can experience hydrodynamic expansion by heating of the stellar XUV (soft X-rays and extreme ultraviolet) radiation and thermal escape via blow-off. Depending on orbit location, XUV flux, heating efficiency and the planet’s mean density our resultsindicate that the upper atmospheres of all “super-Earths” can expand to large distances, so that besides of Kepler-11c all of them experience atmospheric mass-loss due to Roche lobe overflow. The atmospheric mass-loss of the studied “super-Earths” is one to two orders of magnitude lower compared to that of “hot Jupiters” such as HD 209458b, so that one can expect that these exoplanets cannot lose their hydrogen-envelopes during their remaining lifetimes.

Damping of oscillations by ion-neutral collisions in a prominence plasma

Aims. The role of collisions between ions, electrons and neutrals in a partially ionised plasma is assessed as a possible wave damping mechanism. The relevance of this mechanism in the damping of small amplitude prominence oscillations is evaluated. Methods. A one-fluid MHD set of equations taking into account various effects in a partially ionised solar plasma (collisions between different species and Joule dissipation) is derived. Assuming small perturbations, these equations are next linearised about a uniform equilibrium configuration and the dispersion relation of magnetoacoustic waves in an unbounded medium is obtained. Results. The presence of neutrals in the plasma only affects the fast wave in a relevant way. An approximate expression for the damping rate is obtained which shows that the strongest damping takes place in a medium with strong magnetic field, low density and low ionisation fraction. Wave attenuation arises mostly from collisions between ions and neutrals. Conclusions. Given the poor knowledge about the values of the density and ionisation fraction in prominences, it is hard to judge the importance of the physics of partial ionisation in the damping of fast waves in solar prominences. Nevertheless, note that a very idealised case, with no stratification and no equilibrium currents, is considered here, so the addition of these features to the model may change the results of this work.

Magnetohydrodynamic waves in solar partially ionized plasmas: two-fluid approach

Context. Partially ionized plasma is usually described by a single-fluid approach, where the ion-neutral collision effects are expressed by Cowling conductivity in the induction equation. However, the single-fluid approach is not valid for time-scales less than ion-neutral collision time. For these time-scales the two-fluid description is the better approximation. Aims. We aim to derive the dynamics of magnetohydrodynamic (MHD) waves in two-fluid partially ionized plasmas and to compare the results with those obtained under single-fluid description. Methods. Two-fluid equations are used, where ion-electron plasma and neutral particles are considered as separate fluids. Dispersion relations of linear waves are derived for the simplest case of homogeneous medium. Frequencies and damping rates of waves are obtained for different parameters of background plasma. Results. We found that two- and single-fluid descriptions give similar results for low-frequency waves. However, the dynamics of MHD waves in the two-fluid approach is significantly changed when the wave frequency becomes comparable with or higher than the ion-neutral collision frequency. Alfven and fast magneto-acoustic waves attain their maximum damping rate at particular frequencies (for example, the peak frequency equals 2.5 times the ion-neutral collision frequency for 50% of neutral hydrogen) in the wave spectrum. The damping rates are reduced for the higher frequency waves. The new mode of slow magneto-acoustic wave appears for higher frequency branch, which is connected to neutral hydrogen fluid. Conclusions. The single-fluid approach perfectly deals with slow processes in partially ionized plasmas, but fails for time-scales shorter than ion-neutral collision time. Therefore, the two-fluid approximation should be used for the description of relatively fast processes. Some results of the single-fluid description should be revised in future such as the damping of high-frequency Alfven waves in the solar chromosphere due to ion-neutral collisions.

Magnetic moment and plasma environment of HD 209458b as determined from Lyα observations

Transit observations of HD 209458b in the stellar Lyman-α(Lyα) line revealed strong absorption in both blue and red wings of the line interpreted as hydrogen atoms escaping from the planet’s exosphere at high velocities. The following sources for the absorption were suggested: acceleration by the stellar radiation pressure, natural spectral line broadening, or charge exchange with the stellar wind. We reproduced the observation by means of modeling that includes all aforementioned processes. Our results support a stellar wind with a velocity of ≈400 kilometers per second at the time of the observation and a planetary magnetic moment of ≈1.6 × 1026 amperes per square meter. An exoplanet’s magnetic field is manifested as a particular absorption pattern in the transmitted spectrum of the host star. Transit marked by magnetosphere effects Life on Earth exists under the protective sheath of our magnetosphere that deflects charged particles blown out by the Sun. Kislyakova et al. calculated the strength of the magnetic field of a well-studied hot-Jupiter–type exoplanet that produces similar effects. During the planets transit in front of its host star, HD 209458, hydrogen atoms leave a peculiar asymmetric signature in the transmitted spectrum. Science, this issue p. 981

Geophysical and Atmospheric Evolution of Habitable Planets

The evolution of Earth-like habitable planets is a complex process that depends on the geodynamical and geophysical environments. In particular, it is necessary that plate tectonics remain active over billions of years. These geophysically active environments are strongly coupled to a planets host star parameters, such as mass, luminosity and activity, orbit location of the habitable zone, and the planets initial water inventory. Depending on the host stars radiation and particle flux evolution, the composition in the thermosphere, and the availability of an active magnetic dynamo, the atmospheres of Earth-like planets within their habitable zones are differently affected due to thermal and nonthermal escape processes. For some planets, strong atmospheric escape could even effect the stability of the atmosphere.

ATMOSPHERE EXPANSION AND MASS LOSS OF CLOSE-ORBIT GIANT EXOPLANETS HEATED BY STELLAR XUV. II. EFFECTS OF PLANETARY MAGNETIC FIELD; STRUCTURING OF INNER MAGNETOSPHERE

This is the second paper in a series where we build a self-consistent model to simulate the mass-loss process of a close-orbit magnetized giant exoplanet, so-called hot Jupiter (HJ). In this paper we generalize the hydrodynamic (HD) model of an HJ expanding hydrogen atmosphere, proposed in the first paper, to include the effects of intrinsic planetary magnetic field. The proposed self-consistent axisymmetric 2D magnetohydrodynamics model incorporates radiative heating and ionization of the atmospheric gas, basic hydrogen chemistry for the appropriate account of major species composing HJs upper atmosphere and related radiative energy deposition, and H3+ and Ly{\alpha} cooling processes. The model also takes into account a realistic solar-type X-ray/EUV spectrum for calculation of intensity and column density distribution of the radiative energy input, as well as gravitational and rotational forces acting in a tidally locked planet-star system. An interaction between the expanding atmospheric plasma and an intrinsic planetary magnetic dipole field leads to the formation of a current-carrying magnetodisk that plays an important role for topology and scaling of the planetary magnetosphere. A cyclic character of the magnetodisk behavior, composed of consequent phases of the disk formation followed by the magnetic reconnection with the ejection of a ring-type plasmoid, has been discovered and investigated. We found that the mass-loss rate of an HD 209458b analog planet is weakly affected by the equatorial surface field <0.3 G, but is suppressed by an order of magnitude at the field of 1 G.

Variability of solar/stellar activity and magnetic field and its influence on planetary atmosphere evolution

It is shown that the evolution of planetary atmospheres can only be understood if one recognizes the fact that the radiation and particle environment of the Sun or a planet’s host star were not always on the same level as at present. New insights and the latest observations and research regarding the evolution of the solar radiation, plasma environment and solar/stellar magnetic field derived from the observations of solar proxies with different ages will be given. We show that the extreme radiation and plasma environments of the young Sun/stars have important implications for the evolution of planetary atmospheres and may be responsible for the fact that planets with low gravity like early Mars most likely never build up a dense atmosphere during the first few 100 Myr after their origin. Finally we present an innovative new idea on how hydrogen clouds and energetic neutral atom (ENA) observations around transiting Earth-like exoplanets by space observatories such as the WSO-UV, can be used for validating the addressed atmospheric evolution studies. Such observations would enhance our understanding on the impact on the activity of the young Sun on the early atmospheres of Venus, Earth, Mars and other Solar System bodies as well as exoplanets.